This invention relates to an apparatus that forms color images by repeating the process of applying thermal energy imagewise to a colorant sheet having a thin film of colorant and then transferring the imagewise pattern of thin colorant film onto an image-receiving material from the other part of the colorant sheet by peel/transfer. More specifically, the invention relates to a color image forming apparatus that prepares direct digital color proofs for use in the printing area by image recording with lasers or thermal heads in response to digital image signals.
Thermal printers, or apparatus that form images by heat transfer recording, have heretofore been proposed. A thermal head furnished with a number of heat generating elements is pressed against an image-receiving sheet via an ink sheet and the heat generating elements are selectively activated to generate heat in response to an image signal. The ink transfer layer in the ink sheet is discretely softened, melted vaporized or sublimed so that the ink is transferred in dots onto the image-receiving sheet, whereby the image is recorded.
The selective heating of the ink sheet in response to an image signal in the heat transfer recording method may be effected by laser-emitted light of high-density energy and this method has been proposed as "laser recording" in a heating mode.
In heat transfer recording by either a thermal head or a laser operating in a heating mode, color images can be produced by performing multiple transfer recording on an image-receiving sheet from an ink sheet or ink sheets with three colors, yellow (Y), magenta (M) and cyan (C), or four colors, Y, M, C and K (black), to produce one print. If necessary, the time of electric current application to the heat generating elements in the thermal head or to the laser, hence, the time of heat generation by the thermal head or the time of light emission from the laser, may be controlled so as to modulate the amount or area of the ink that is to be softened,or melted or which is to be vaporized or sublimed, whereby either the ink density or the area of halftone (dots) is modulated to produce density gradations in individual pixels.
The basic layout of a prior art apparatus for forming images by the above-described method of heat transfer recording is shown in FIG. 5. The apparatus generally indicated by 100 in FIG. 5 performs heat transfer image recording by means of a thermal head 112. The image-receiving sheet 102 is held by a clamper (not shown) onto a timing belt 106 that is wound around a belt drive roller 103 that is motor driven, a platen roller 104 and an idle roller 105 and it is transported as the platen roller 104 rotates.
An ink sheet 110 is unwound from a supply roll 111 and brought into a superposed relationship with the image-receiving sheet 102 on the platen roller 104. The ink sheet is thereafter heated imagewise by means of a thermal head 112 provided in close proximity to the platen roller 104, whereupon the ink layer in the ink sheet 110 melts or sublimes to be transferred imagewise onto the image-receiving sheet 102 for image recording. Subsequently, the used ink sheet 110 is wound up by a takeup roll 113 and the image-receiving sheet 102 is returned to the recording start position. This image recording procedure is taken as many times as are required to record three or four colors (including the number of times required for any special colors to record in addition to Y, M, C and K).
FIG. 6 shows another prior art apparatus for recording images by heat transfer with a thermal head. The apparatus generally indicated by 120 uses an image-receiving sheet 122 that is not a cut sheet but a continuous web. Being unwound from a supply roll 124, the sheet 122 contacts a platen roller 126 and is wound up by a takeup roll 128. As in the case shown in FIG. 6, an ink sheet 130 is unwound from a supply roll 132, brought into a superposed relationship with the image-receiving sheet 122 on the platen roller 126 and thereafter heated imagewise with a thermal head 134 provided in close proximity to the platen roller 126. Upon heating, the ink layer on the ink sheet 130 melts or sublimes and is transferred onto the image-receiving sheet 122 for image recording. The used ink sheet 130 is wound up by a takeup roll 136. Thereafter, the image-receiving sheet 122 is reversed to the recording start position and the same procedure is repeated for image recording in a next color. This image recording procedure is taken as many times as are required to record three or four colors, thereby forming a full color image.
In both the heat transfer image recording apparatus 100 and 120 shown in FIGS. 5 and 6, respectively, the thermal head 112 or 134 may be replaced by a laser head capable of emitting laser light of high energy density and such a method of recording in a heating mode may also be adopted.
If desired, both the image-receiving sheet and the ink sheet may be cut sheets that are wound onto a recording drum for performing thermal recording. This can be done effectively whether a thermal head or a laser head operating in a heating mode is employed.
A fusion-type thermal transfer image recording apparatus that depends on a laser head operating in a heating mode is described in Unexamined Published Japanese Patent Application (kokai) Hei 5-254188. The apparatus has a hold-down means for preventing the image-receiving and ink sheets from wrinkling or otherwise deforming when they are held by suction on the recording drum, thereby enabling the sheets to be fed and ejected automatically.
The ink sheets 110 and 130 used in the conventional thermal transfer image recording apparatus are either softenable to melt the ink or vaporizable to have it sublime. The first type of ink sheets comprises a support carrying a transfer layer (image forming layer) that has a colorant mixed in a binder which is selected from among waxes and other low-melting point substances that will soften or melt upon heating. The second type of ink sheets is characterized by the use of a binder that sublimes upon heating. Hence, the thickness of the transfer layer is at least 5 .mu.m and, sometimes, as great as 20-30 .mu.m irrespective of whether the ink sheets are of the melt or sublimation type. Because of this thickness problem, great energy is required to heat the transfer layer until the ink sublimes or melts, or the image resolution cannot be increased beyond a certain level or small dots cannot be reproduced consistently on account of blurred or jagged edges.
In the conventional method of image recording by heat transfer, the colorant or any other necessary components will readily transfer from the ink sheet to the image-receiving sheet as a result of the melting or sublimation of the transfer layer in the ink sheet and, hence, the ink sheet need only to be brought into intimate contact with the image-receiving sheet without causing wrinkles or other deformations in the prior art models 100 and 120. In addition, the ink sheet and the image-receiving sheet which are merely placed in intimate contact with each other can be readily separated as required. Under these circumstances, no consideration has been given to the means of joining the two sheets with a uniform adhesive force or the means of separating them without causing unevenness.
When bonding the two sheets together by an adhesive force, a displacement or positional effect may occur in the sheets to be joined; alternatively, the image-receiving sheet may be displaced when the ink sheet is peeled therefrom in counteraction against the adhesive force. This problem with the lower precision of registration has not been taken into account in the prior art. The hold-down means used by the image recording apparatus described in Unexamined Published Japanese Patent Application (kokai) Hei 5-254188, supra, merely holds the image-receiving sheet and the ink sheet under their own weight on the recording drum. This is effective in preventing the occurrence of wrinkles but, on the other hand, it is not applicable to the case where there is the need to bond the two sheets together by a uniform adhesive force.